Chapter 4

Precise Delay Alignment Between Amplitude and Phase/Frequency Modulation Paths in a Digital Polar Transmitter

KHURRAM WAHEED and ROBERT BOGDAN STASZEWSKI

Texas Instruments, Dallas, Texas

4.1 INTRODUCTION

Modern complex envelope modulation schemes such as EDGE, WCDMA, extended-data-rate Bluetooth (BT-EDR), WLAN, LTE, and WiMAX impose strict performance requirements on radio-frequency (RF) transceivers. For digital polar transmitters, typical stringent performance requirements exist for modulated close-in and far-out spectra, adjacent channel power/leakage ratio (ACPR/ACLR), error vector magnitude (EVM), phase trajectory error (PTE), and percentage power in-band. Implementation of such modern communication standards using the digital polar modulation approach is possible only if precise alignment can be maintained between the amplitude modulation (AM) and phase/frequency modulation (PM/FM) paths. This is an arduous task, as both the amplitude and phase (or frequency) paths have different modulation bandwidths and comprise digital components that need to operate on different clock domains for power efficiency, while complying with the stringent performance requirements. The front end comprises digitally controlled analog components, such as a digitally controlled oscillator (DCO) and a digitally controlled pre-power amplifier, which together transform the digital signals to the continuous-time domain with high precision.

Figure 4.1 is an overview of key circuits of a single-chip ...

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